1. Field of the Invention
This invention relates to a rotating viscometer capable of accurate measurement of fluid viscosity for both Newtonian and non-Newtonian fluids, and which is operative with small amounts of such fluids. The rotating viscometer is suited especially for use with a screen printing machine and is designed in such a way that it allows measurement and control of the viscosity of a fluid, such as paste, immediately before it is coated over a substrate by the printing machine. The viscosity of such fluids can be specified and precise control over ,the viscosity of the fluid can be achieved by maintaining the fluid shearing time constant while continuously kneading the fluid. As a result of these features, the rotating viscometer of this invention enables accurate screen printing.
2. Description of the Prior Art
There is an ever-increasing demand for the densification and hybridization of electronic devices in this age of electronics. According to the hybrid microelectronics technology which has been developed to meet that demand, active elements equipped with various functions are packaged in combination with passive elements such as resistors, capacitors and the like on a substrate to materialize a light, thin, short and small hybrid IC module.
In the fabrication of a hybrid IC modules, a substrate is first printed using a screen printing machine. The substrate is printed in this manner with various pastes such as conductive paste, resistive paste, dielectric paste and solder paste, a resist ink, and inks of other functional materials. This printing work requires a high degree of accuracy because of the nature of the hybrid IC module. In such printing work, the accuracy of printed film thickness and patterns is heavily dependent on the accuracy of control of various functions of the screen printing machine.
A controlled variable which is considered to be most important to increased accuracy of printing, but which is regarded as a variable difficult to control, is the viscosity of a paste (ink), itself. As a result, the difficulties in controlling the viscosity of the paste are dealt with by relying on the experience of printing technicians and laboratory measurements in may instances.
The difficulties encountered in controlling the viscosities of pastes can be attributed to the fact that many pastes, behave as non-Newtonian fluids, have thixotropic properties and are hence affected by surrounding conditions such as temperature and humidity. Moreover, the viscosities of such fluids do not remain constant either during or after kneading. Even if the viscosity of a paste is measured by a viscometer after its kneading, the paste is fed to a screen printing machine subsequent to its kneading to a desired viscosity and a substrate is then printed with the-thus fed paste, the viscosity of the paste changes moment by moment and does not remain stable during the printing work so that no accurate screen printing work is feasible. Moreover, the paste is kneaded further by a squeegee (i.e., a doctor blade for coating the paste) during the printing work. This additional kneading renders the viscosity of the paste more unstable, thereby making it difficult, if not impossible, to perform accurate screen printing.
Viscometers which have conventionally been employed for screen printing are rotating viscometers, because they are also employed to knead pastes stored in tanks. Specifically, the rotational resistance of each rotating viscometers is recorded as a viscosity. Because of the kneading work performed, the conventional viscometers are relatively large and expensive. Also in the use of such rotating viscometers, the viscosity of a paste in a tank is measured as if it was done in a laboratory. It is impossible to measure the viscosity of a paste at the point of application by a screen printing machine. Since the viscosity of the paste cannot be measured and controlled accurately, no precise screen printing can be expected.
As mentioned above, the pastes used in the fabrication of hybrid IC modules have properties of non-Newtonian fluids so that the viscosities of the pastes vary considerably depending on temperature. In addition, the viscosity, of the paste changes significantly in accordance with the speed and time of shearing the paste by the rotary member. For these reasons, the measurement of the paste viscosity in the past was periodically effected at a fixed interval, for example, at every third minute by using a stopwatch. While this approach to viscosity measurement may appear to be accurate, it requires measurement of the viscosity of the whole of a large amount of a paste being kneaded. Otherwise, its viscosity cannot be accurately determined. In addition, the viscosity must be measured repeatedly many times. The above method is hence accompanied by further drawbacks that the viscosity of the paste cannot be measured continuously, the measurement of the viscosity cannot be automated, the efficiency of the measurement is low, and measurement of the viscosity is cumbersome.
Thus a major problem in the measurement and regulation of paste viscosity in prior methods for fabricating hybrid IC modules is that the shearing time cannot be maintained constant. In this respect, it has been attempted to measure the viscosity of a paste while recirculating the paste forcibly through the shearing gap of rotating double-cup-type viscosimeter. This approach, however, (1) requires complex system for the forced recirculation, leading to excessive viscosimeter and expense; (2) the recirculating force affects the torque produced to measure the viscosity of the paste, thus reducing accuracy of the viscosity measurement; (3) when gravity forced recirculation is employed, the shearing time varies; and (4) the forced recirculation method requires a larger flow passage for the recirculation of the paste, again leading to the drawback that the rotating viscometer itself becomes large and expensive.